1. Field of the Invention
The present invention relates generally to field emission cathodes and methods of cleaning therefor.
2. Description of the Related Art
Electron sources or cathodes are essential to the functioning of all electron cathodes. Traditionally, cathodes for vacuum cathodes such as vacuum tubes and cathode ray tubes used thermionic emission to produce the required electrons. This required raising cathode materials to very high temperature conduction of current either by direct or through the use of auxiliary heaters. The process is very inefficient, requiring relatively large currents and dissipating most of the energy as wasted heat.
In recent years there has been a growing interest in replacing the inefficient thermionic cathodes with high field emission cathodes. These cathodes are very efficient because they eliminate the need to heat the cathode material. They have been used for a number of years as sources for scanning electron microscopes, and are now being investigated as sources for vacuum microelectronic cathodes, flat panel displays, and high performance high frequency vacuum tubes.
Field emission cathodes include very sharp points of field emission materials. These sharp points when biased with negative potential concentrate the electric field at the point. This high electric field allows the electrons to "tunnel" through the tip into surrounding space which is normally maintained under high vacuum conditions. The magnitude of the potential required to produce sufficiently strong electric fields is proportional to the distance between the tip and the principal extraction electrode. This principal extraction electrode will be referred to as the extraction electrode. While this extraction electrode can be a physically separate structure, minimum extraction potentials can most conveniently be obtained by physically integrating the extraction electrode directly with the field emission tips. This produces very small extraction electrode-cathode distances, which are physically locked in proper alignment. Field emission cathode structures both with and without integrated extraction electrodes are useful electron sources in a variety of current and potential applications such as displays, Vacuum Microelectronic Cathodes, and various electron microscopes.
The field emission display elements that utilize these cathodes use the basic field emission structure and add additional structures, such as, an extension of the vacuum space, a phosphor surface opposite the cathode tip, and additional electrodes to collect and/or control the electron current. Groups of individual Vacuum Microelectronics Cathodes and/or display elements are electrically interconnected during fabrication to form integrated circuits and/or displays.
While these field emission cathode structures can be made in almost any size and may have application as discrete sources, their best performance and major application is expected to come from extreme miniaturization, and dense arrays.
Non-thermionic field emitters, field emission cathodes, and field emission displays are all known in the art. The fabrication of the field emission cathode structure is a critical element common to the cathode mentioned. The material (insulators and conductors/field emitters) are all deposited and processed by relatively common deposition and lithographic processing techniques with the single exception of special sharp edge (blade) or point (tip) structure which is common to all field emission cathodes.
The art of fabricating the sharp field emission tip or blade can be classified into several categories.
In one of the earliest categories, the cathode tip structure is formed by the direct deposition of the material. An example of this type is described in a paper by C. A. Spindt, "A thin-Film Field-Emission Cathode", Journal of Applied Physics Vol. 39, No. 7, page 3504 (1968). In this paper, sharp molybdenum cone-shaped emitters are formed inside holes in a molybdenum anode layer and on a molybdenum cathode layer. The two layers are separated by an insulating layer that has been etched away in the areas of the holes in the anode layer down to the cathode layer. The cones are formed by simultaneous normal and steep angle depositions of the molybdenum and alumina, respectively, onto the rotating substrate containing the anode and cathode layers.
In this developing field, the art has produced closely packed arrays of such cones. One of the fabrications of arrays of cones is exemplified in a paper by C. A. Spindt, I. Brondie, L. Humphrey, and E. R. Westberg, `Physical properties of thin-film field emission cathodes with molybdenum cones", Journal of Applied Physics, Vol. 47, No. 12, pages 5248 to 5263 (1976).
In this paper, field emission cathodes fabricated using thin-film techniques and microlithography are described, together with effects obtained by varying the fabrication parameters. The emission originates from the tip of molybdenum cones that are about 1.5 .mu.m tall with a tip radius around 500 angstroms. Such cathodes have been produced in closely packed arrays containing 100 and 5000 cones as well as singly. Minimum currents in the range 50-150 .mu.A per cone can be drawn with applied voltages in the range 100-300 V when operated in conventional vacuum at pressures 10.sup.-9 Torr or less. In the arrays, current densities (averaged over the array) of above 10 A/cm.sup.2 have been demonstrated.
These field emission cathodes and extraction electrodes can be used in practical applications, such as flat-panel displays. In the display applications, beam focusing is required as is discussed in a paper by W. Dawson Kesling, and Charles E. Hunt entitled "Beam Focusing for Field-Emission Flat-Panel Displays", IEEE Transactions on Electron Cathodes, Vol. 42, No. 2, pages 340 to 347, February 1995. In the paper, two focused designs have been shown to yield exceedingly small beam widths. One design uses an aperture electrode parallel to the gate electrode on the cathode substrate for beam focusing. The other is a concentric electrode design, in which the focus electrode lies in the same plane as the gate electrode and surrounds each emission tip.
In the practical applications, it is necessary to avoid disruption of the cathode in life. This disruption is caused by a local gas discharge forming between the tips and the gate electrode. Gas out of the active components of the cathode, including the cathode itself, is the main source of gas for this discharge. It is known that bombarding all the active parts of the cathode (including the cathode) with electrons can eliminate this effect. The several bombarding techniques have been described in JP-A 4-22038, JP-A 5-198255, and JP-A 114353.
JP-A 4-220388, which apparently corresponds to U.S. Pat. No. 5,189,341, describes electron-bombarding technique. A plurality of pairs of cathodes separated from each other is used. A portion of electrons from emitter tips on one cathode of each pair is drawn toward emitter tips on the other cathode for electron bombardment when the emitter tips on the other cathode do not emit electrons. A portion of electrons from the emitter tips on the other cathode is used for bombardment of the emitter tips on the one cathode.
JP-A 5-198255, which appears to correspond to EP-A 0 541 394, describes an electron-bombarding technique in which an anode electrode is used to direct substantially all of electrons from emitter tips on one cathode to emitter tips on the adjacent cathode.
JP-A 5-114353 shows, a plurality of emitter tips surrounding an emitter tip on a cathode. The surrounding emitter tips emit electrons for bombardment of the emitter tip surrounded thereby. The surrounding emitter tips are formed on an extraction electrode of the surrounded emitter tip. Thus, the extraction electrode is negatively biased for urging the surrounding emitter tips to emit electrons and the cathode of the surrounded emitter is positively biased to draw the electrons. According to this arrangement, positive voltage applied to the cathode cannot be increased to a sufficiently high level because the extraction electrode is negatively biased. This is because the difference in potential between the surrounded emitter tip and the extraction electrode must be low enough not to induce electrical discharge across a gap between them.
JP-A 60-1741 describes an electron gun in which a filament is opposed to an anode to heat the surface of the anode during a gas discharge process thereby facilitating gas discharge from the anode. In operation of the cathode, the filament is positively biased thus preventing ions separated from the surface of the anode from leaving therefrom.
An object of the present invention is to provide a field emission cathode that makes it possible and easy to clean not only emitter tips but also electrodes by electron-bombarding during discharge process before sealing a space around the emitter tips and electrode.
A specific object of the present invention is to provide a field emission cathode that allows beam focusing to yield small electron beam widths after electron-bombarding of electrodes as well as emitter tips during discharge process.
Another object of the present invention is to provide a method of cleaning a field emission cathode during gas discharge process before sealing a space around emitter tips and electrodes.